The separation of gases by differential gas permeation through a polymeric membrane is a commercially recognized technique that continues to grow in importance. Presently, membrane systems are used to separate carbon dioxide/methane, oxygen/nitrogen, hydrogen/nitrogen, helium/nitrogen and the like gas mixtures. Other gases that might also be separated using this technique include helium/methane, ethylene/ethane, propylene/propane, nitrogen/methane and the like mixtures.
Gases produced by differential permeation find many applications. For example, nitrogen generated by differential permeation through a membrane is particularly useful for blanketing reactors and storage vessels, for use on offshore platforms and in marine tankers, for purging and pressurizing pipelines and tanks as well as for drying reactive chemicals. Other uses include fruit and vegetable storage under controlled atmospheric conditions to lengthen product life, optimally with a 95-98% nitrogen blanket at a temperature slightly above freezing. Oxygen generated by differential permeation through a membrane is useful for medical applications, enriching air streams to enhance combustion, enriching air for organic waste treatment, and the like.
The polymers currently used to produce membranes for gas separation applications are mainly those that provide membranes that need no further modification or treatment. Polymers presently employed commercially are primarily amorphous and glassy such as polysulfones, polyimides, and cellulosics. Crystalline, non-glassy polymers have not generally been considered optimally useful for gas separation applications because of their limited separation capabilities, particularly compared to amorphous, glassy polymers such as polysulfones and polyimides. Nevertheless, some other polymers have been observed to exhibit interesting separation characteristics. Poly(4-methyl-1-pentene), for example, has been commercialized in melt spun hollow fiber form for oxygen/nitrogen separations to produce nitrogen and oxygen useful primarily for medium purity nitrogen (95-97%) and enriched oxygen air applications, respectively. A commercial system using poly(4-methyl-1-pentene) is discussed in a review by Fritzsche et al, Gas Separations by Membrane Systems, Chemical Economy and Engineering Reviews, 19 (1, 2, 3), 19 (1987). This article also reviews polymeric membrane gas separation systems and gives an excellent summary of the technology, applications, and polymeric membranes employed for such applications. Other publications describe, for example, polyolefin-based hollow fiber membranes used in a commercial units to separate oxygen and nitrogen and also for other gas separations; i.e., Stannett et al, Recent Advances in Membrane Science and Technology, Adv. Polym. Sci., 32, 69 (1979); Stern et al, Tests of a Free-Volume Model for the Permeation of Gas Mixtures Through Polymer Membranes CO.sub.2 --C.sub.2 H.sub.4, CO.sub.2 --C.sub.3 H.sub.8, and C.sub.2 H.sub.4 --C.sub.3 H.sub.8 Mixtures in Polyethylene", J. Polym. Sci., Polym. Phys. Ed., 21, 1275 (1983); and Robeson et al, Permeation of Ethane-Butane Mixtures through Polyethylene, J. Appl. Polym. Sci., 12, 2083 (1968).
Some surface modification techniques have been proposed to provide enhanced membrane selectivity without greatly reducing the throughput of the system. Such proposed treatments include UV exposure, plasma treatment, plasma polymerization, and fluorination. Osterholz, in U.S. Pat. No. 3,846,521, teaches a low energy electron beam treatment for polymeric films, including poly(4-methyl-1-pentene). Kipplinger et al (J. Appl. Polym. Sci., 31, 2617 (1986) observed improved separation properties for fluorinated low density polyethylene, and Langsam (U.S. Pat. No. 4,657,564) discloses that the fluorination of poly(trimethylsilylpropyne) membranes produces significant increases in the selectivity for a number of gas pairs including oxygen/nitrogen, helium/methane, hydrogen/nitrogen, helium/nitrogen, hydrogen/methane, carbon dioxide/methane, and the like. The reported treatments produce an extremely thin membrane surface layer, usually less than a micron, which determines the separation characteristics of the membrane. Consequently, surface modification can render relatively thick and easy-to-obtain dense films useful for gas separation purposes without need for applying ultra-thin coatings.
Dixon, U.S. Pat. No. 4,020,223 teaches subjecting fiber form synthetic resins such as polyolefins and polyacrylonitriles to a fluorination treatment along with low levels of elemental oxygen to impart stain release properties to the fibers.